One of the reasons I don't trust the police is because they cost a childhood friend his place studying medicine at university.
Case of wrongful identity at an "unders" (thats <=18 years of age for people not familiar with it) gig saw him going to court on "sexual assault of a minor" because he was 18 and in the club on the same night. CCTV footage showing my friend entering the club more than an hour after the offence was commited, and the victim (after the arrest of course, bless her) confessed that she hadn't gotten a good look at the guy and was pretty sure it wasn't my friend.
However, current policy meant so many of these cases had to go to court and the date set happened to be my friend's Advance Higher Chemistry exam. Despite begging, pleading and all other forms of grovelling, the date was set in stone and he had to go. Case was dismissed almost immediately but he had to resit the exam. That resit cost him his place. Try explaining at the interview stage that you had to resit due to being in court. Goes down like a bloody lead balloon.
Now I can appreciate there was no maliciousness in this on the police's part, but it does blow a rather large whole in "The innocent have nothing to fear from the police".
I've had two run ins myself with the police, both on the road. In the first, the two police officers were a credit to their profession and they really helped me out. I'd skidded on surface water and had had quite a bad crash. They couldn't have been more decent about it. They helped me calm down, helped me sort out the car and even gave me a lift to where I was going.
The second was a bloody nightmare. I was pulled over because the arresting officer believed the car I was driving was stolen. I was questioned, insulted, cuffed and chucked in the back of the police car. After about half an hour of waiting for the recovery vehicle to come lift my "stolen car" the police finally gave in to my pleas and rechecked the report on my apparent car theft. Turns out it was a different car. The registration was similar (2 digits different to mine) but the colour and model were also different! I was uncuffed and let out of the vehicle, which then drove away while I was still standing on the side of the motorway. No apology was given and they didn't even wait for me to get back in my vehicle before driving off.
I do not hate the police as a result of this, but I sure as hell don't find them particularly comforting when you never know if your going to get the good ones or the bad ones. I feel sorry for good officers that have their reputations sullied by the misdemeanours of their fellows, but I have a lot more sympathy for the people that just get the s**t end of the stick when dealing with them.
This was all in Scotland in the UK (for reference).